I know some basic things from Google but I can’t understand why should I change the aspect ratio if I can change it by modifying the pixel ratio. If I don’t change the pixel ratio but change the aspect, then its distort everything. Thanks in advance
Don’t worry about aspect ratio unless you know what you’re doing. If you’re working with real-world footage, a lot of cameras use a non-1:1 aspect ratio that you have to match in Blender. If you aren’t, leave it at 1:1. That is to say- if you don’t already know what the aspect ratio you want to use is (you weren’t given it by a camera person or TD), use 1:1 
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Concerns such as “aspect ratio” would apply to any image that you intend to “capture and then present,” no matter what technology you intended to use – film, digital camera, CG. (This becomes vitally important whenever you need to merge your CG “footage” with that of any actual camera.)
Whereas, “pixel ratio” is entirely linked to one particular presentation technology, and it is dependent upon the physical characteristics of "this device." So, if for whatever reason you then need to “do it again,” you have to “do it over.”
So, I encourage you to – when appropriate – “treat CG like a camera.” Let Blender do the work for you.
Although, as always, “TMTOWTDI = There’s More Than One Way To Do It,™” some ways are better than others.
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Thanks everyone for comment!
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Yeah, aspect ratio is kind of a pain from the ancient times !
Mainly for technical reason ( to save bandwith) or to fit a widescreen image in a 4:3 film people invented anamorphic film.
It was also used in video at the very beginning of digital camera.
But nowadays it’s less and less used since we found different ways to address these issues.
And cinema projectors are digital as well. Bye bye movie film !
It’s quite common to still film movies in anamorphic since it do very interesting effect on the image, giving an instant cinematic look.
But I’m pretty sure once the shooting is done they try to go back to the final format and work in ratio 1:1 ASAP.
So yeah you can forget about these buttons but studying the subject, and especially anamorphic lenses can lead you to interesting knowledge especially if you’re into cinematic rendering !